1. Field of the Invention
The present invention relates to a multistage variable gain amplifier circuit suitable for radio-frequency amplifying circuits of, for example, a CDMA (Code Division Multiple Access) mode portable telephone.
2. Description of the Related Art
In a CDMA mode portable telephone, variable gain amplifying circuits (hereinafter called "variable amplifying circuits") capable of varying a gain of 80 dB or above are normally provided at radio-frequency amplifying circuits of a transmitter unit and a receiver unit, respectively, in order to maintain communication when the portable telephone moves. FIG. 4 shows a radio frequency stage of a portable telephone having general CDMA and FM dual modes. A description will first be made of a configuration of a transmission (TX) system. An IF (intermediate frequency) transmit signal modulated by a modem 101 is QPSK-modulated by a QPSK modulating circuit 102. Next, the modulated signal is amplified by a transmitter variable amplifying circuit (TX-AMP) 103, which in turn is mixed into a local oscillating frequency produced from a local oscillator (OSC) 121 by a mixer (MIX) 104, where the mixed signal is converted into an RF (radio frequency) transmit signal. The RF transmit signal is transmitted through a band-pass filter 105, a power amplifier (PA) 106, a duplexer 107 and an antenna 108.
A description will next be made of a configuration of a reception (RX) system. An RF receive signal received by the antenna 108 is applied to a mixer (MIX) 111 through the duplexer 107, a low noise amplifier (LNA) 109 and a band-pass filter 110, where the signal is mixed into a local oscillating frequency generated from the local oscillator (OSC) 121 converting the signal into an IF receive signal. The IF receive signal is applied to a CDMA band-pass filter 112 and an FM band-pass filter 113, where one output signal is selected according to a set mode and amplified by a receiver variable amplifying circuit (RX-AMP) 114. Next, the amplified signal is demodulated by a QPSK demodulating circuit 115, and then provided to the modem 101.
The strength of the received signal, which has been detected by a receive-signal strength indicating circuit (RSSI) 116 provided within the modem 101 is compared with strength reference data by a comparator 117. The difference in strength therebetween is applied to a receiver AGC voltage correction circuit 118 and a transmitting output correction circuit 119. The AGC voltage correction circuit 118 outputs an AGC output so that the difference produced from the comparator 117 becomes "0," i.e., the output of the RSSI 116 coincides with the strength reference data, thereby controlling the gain of the variable amplifying circuit (RX-AMP) 114 on the receiving side. The difference produced from the comparator 117 and transmitting output correction data determined according to circuit conditions between a portable telephone and a base station are applied to the transmitting output correction circuit 119 on the transmitting side. An AGC voltage correction circuit 120 on the transmitting side outputs an AGC voltage so that a modulated signal is inversely proportional to the level of the received signal, and according to the transmitting output correction data, thereby controlling the gain of the variable amplifying circuit (TX-AMP) 103 on the transmitting side.
In this case, excellent linearity is required between the AGC voltage and the gain over a dynamic range of 80 dB or above in order to allow the variable amplifying circuits 103 and 114 on the transmitting and receiving sides to operate in interlock with each other. Since the portable telephone is driven by a battery, the battery power decreases as current consumption increases. As a result, a problem arises in that, for example, standby time and call duration are reduced and the battery must be replaced frequently. It is thus desired that the variable amplifying circuits 103 and 114 also provide current consumption as low as possible.
There are known current constant mode and current variable mode variable amplifying circuits whose differential amplifiers are respectively connected to constant-current sources. The range in which the gain can be linearly controlled, is normally about 20 dB to 30 dB in a variable amplifying circuit of one stage. A method of cascade-connecting variable amplifying circuits of the same type in three to four stages on a radio-frequency basis and parallel applying an AGC voltage to the respective variable amplifying circuits allows a dynamic range of gain of 80 dB or above.
FIGS. 5 and 6 respectively show commonly-used current constant mode and current variable mode variable amplifying circuits each composed of bipolar transistors. Symbols IN, OUT, VAGC and Vcc denote an input, an output, an AGC voltage and a power source voltage respectively. FIG. 7 shows the gain PG with respect to the AGC voltage VAGC, in which g represents a characteristic of the current constant mode variable amplifying circuit and h presents a characteristic of the current variable mode variable amplifying circuit.
In the current constant mode shown in FIG. 5, gain variable transistors Q1, Q2, Q3 and Q4 and amplifying transistors Q5 and Q6 constitute a differential amplifier. Further, resistors R1 and R2 are respectively load resistors for the transistors Q1 and Q4, E1 is a bias supply or source, and CS1 is a constant-current source.
The gain PG dB! of the current constant mode shown in FIG. 5 is represented by the following expression: EQU PG.varies.PGO+20 log (I1/I0) (1)
where PGO indicates gain at the time that I1 is I0. Further, the relationship between I1 and I0 (I1/I0) is represented by the following expression: EQU I1/I0.varies.1+exp{-VAGC*q/(kT)}! (2)
where
q: unit charge of electron PA1 k: Boltzmann constant, and PA1 T: absolute temperature. PA1 PG.varies.VAGC
The gain PG of the characteristic g does not change linearly when the AGC voltage VAGC is large. In the current constant mode, the amount of tertiary distortion caused by interference or disturbing waves is kept constant regardless of the magnitude of the gain PG. Further, the current consumption is also held constant regardless of the magnitude of the gain PG. In the variable amplifying circuit of a type wherein the current constant mode shown in FIG. 5 is connected in multistage form, the relationship between input intercept points and current consumption with respect to the gain PG is represented in the form of characteristics a and b in FIG. 3. The input intercept point characteristic a in a low range of gain PG is but the current consumption characteristic b is constant.
The current variable mode shown in FIG. 6 comprises gain variable transistors Q7 and Q8, a transistor Q15 for a constant current circuit, and load resistors R3 and R4 for the transistors Q7 and Q8. The gain PG dB! of the current variable mode is represented by the following expression: EQU PG.varies.20 log (I2) (3)
Further, I2 is represented as follows: EQU I2.varies.exp{VAGC*q/(kT)} (4)
Substituting the expression (4) in the expression (3) yields:
As indicated by the curve h in FIG. 7, the gain PG changes linearly with the AGC voltage VAGC. In the variable amplifying circuit of a type wherein the current variable mode is connected in multistage form, the relationship between input intercept points and current consumption with respect to the gain PG is represented in the form of characteristics c and d in FIG. 3. Since a collector current is small, distortion is apt to appear.
However, the variable amplifying circuit in which the current constant mode shown in FIG. 5 is connected in multistage form, has a problem in that the current consumption is large as compared with the current variable mode as indicated by b and d in FIG. 3.
Further, the variable amplifying circuit in which the current variable mode shown in FIG. 6 is connected in multistage form, has a problem in that although the current consumption is small as compared with the current constant mode, the input intercept points deteriorated in the low range of gain PG as compared with the current constant mode as indicated by a and c in FIG. 3. As a result, when a strong electric field is present, other stations will interfere with the corresponding circuit.
In a configuration in which the current constant mode and the current variable mode are cascade-connected, the current constant mode is different in AGC voltage VAGC from the current variable mode. Further, they are different from each other in the characteristic of the gain PG with respect to the AGC voltage VAGC. Therefore, the linearity of gain PG deteriorates.